A vibrating plate compactor of the type referred to above is disclosed, for example, in German published patent application DE-OS No. 32 40 626. In this vibrating plate compactor, the eccentric element comprises two counter-rotating eccentrically loaded shafts, the phase position of which relative to each other is changed by means of a hydraulic system. On changing the phase position, the direction of the vibration force is also changed and with it the direction of translational motion of the vibrating plate compactor. The eccentrically loaded shafts are rotatably joined by means of gears so that when in rotation, the shafts-always tend to assume a definite phase position relative to each other. In this position, a centrifugal force is generated in a definite direction and as a rule the system for controlling the eccentric shafts is so arranged that a forward motion is imparted to the plate compactor at one of the end positions of the system.
Resetting the eccentrics to a position different to the one they tend to assume requires a comparatively powerful adjusting force, especially in the case of large eccentric elements. In the known configuration, this is accomplished by means of a hydraulic servo system so that the operating force applied to the forward-reverse control can be disregarded.
The phase position of the eccentrically loaded shafts relative to each other is controlled by means of a hydraulic piston which is arranged to move axially inside one of the tubular eccentrically loaded shafts and which, by means of a pin fastened to it and running in a spiral groove, causes the tubular eccentrically loaded shaft to rotate when the piston is moved in an axial direction. In the known configuration, the spiral groove is so arranged that the piston can be moved to two end positions where the vibrational force generated by the eccentrics imparts to the vibrating plate compactor a maximal forward and reverse motion, respectively. Even though the servo circuit reduces the force required for adjustment at the control lever, the lever must be held in the position corresponding to reverse motion since the moment of force from the rotating eccentrics would otherwise force them to assume a position corresponding to forward motion of the vibrating plate compactor.